1. Field of the Invention
The invention relates in general to plastic hinges and, in particular, to an improved injection molded plastic elongated hinge adapted for use as an alternative to a conventional piano hinge. Elongated plastic hinges according to the present invention have open spines with spaced apart pivot connections. A substantial amount of material is saved as compared to a conventional piano hinge with a full spine. The assembly of the elongated open spine hinges to hinged members is facilitated. The molding costs are reduced. Both the manufacturing and maintenance of the mold are simplified, and the costs of manufacturing and maintaining the mold are reduced. The cycle time necessary to mold the parts of the open spine hinge is substantially reduced. The open spine hinge can also include a snapped together pivotal connection, if desired.
2. Description of the Prior Art
In many industries, an increasing number of parts and articles are made from plastic. In many instances, these parts are made entirely of plastic. There is also an increasing need for all plastic containers having hinged covers, such as, for example, in the computer industry, toy industry, hardware industry, retail display industry, and the like. In general, where lightweight plastic hinged members such as, for example, flat, transparent, semi-rigid panels are hingedly mounted to containers it is preferred to use continuous plastic hinges such as piano hinges. Continuous hinges generally extend substantially the full length of the edge of the hinged members to which they are mounted. Continuous hinges lend rigidity to the hinged members so that, among other things, the hinged members are easier to assemble and operate, and lighter and less expensive to produce.
Competition in supplying continuous plastic hinges is intense. Profit margins resulting from the mass production of such plastic hinges are very slim, often measured in fractions of a cent per part. Thus, reducing the cost of producing one hinge by just a fraction of a penny can have a significant impact on the economics of producing such plastic hinges. Those skilled in the art recognize that there is a need to mass-produce continuous plastic hinges at the lowest possible cost, consistent with providing a satisfactory product.
Many designs require the use of a continuous hinge configuration commonly known as a xe2x80x9cpiano hingexe2x80x9d, particularly when the members to be joined by the hinge are not strong enough or tough enough to fully support themselves or sustain abuse during rough use. Such use includes, for example, the rotatable joining of relatively thin transparent plastic panels as hinged covers on display cases or containers. Piano hinges are often used to supportably hinge other elongated semi-rigid panels or members. In general, such piano hinges are adapted to extend substantially the full length of an edge of at least one of the hinged members to which it is mounted. Generally, a conventional piano hinge is an elongated hinge wherein the two leaf members of the hinge have a substantial plurality of knuckle or lug portions arrayed along the adjacent edges of the respective leaf members that, when assembled together with a common pivot pin, provide a pivot connection between the two leaf members. The spaced apart knuckles or lugs on each edge define generally cylindrical channels that taken together define an elongated channel. The knuckle or lug portions are located on the edge of each hinged member with the knuckles or lugs on one edge being offset from those on the other. Thus, when the respective edges of the hinged members are brought together the knuckles are interposed or intermeshed. When the knuckle portions are intermeshed and pivotally engaged by a long hinge pin extending generally coaxially through the combined elongated channel, they enable the hinged members to rotate about a hinge axis which is generally coextensive with the longitudinal axis of the hinge pin. The leaf members provide structural support and transfer associated loads through the long hinge pin connection. In contrast, a typical hinge configuration, where the hinge does not extend substantially the full length of the edge of the hinged member, is generally short and will only have a total of 2 or 3 knuckle portions pivotally engaging the leaf members. Generally such hinges do not provide the structural support which is provided by piano hinges. Thus, they generally serve a somewhat different and more limited function. Short hinges, for example, butt or mortise hinges, are often mounted in pairs, the individual ones of which are spaced apart along the edges of the hinged members by a considerable distance. As those skilled in the art recognize, the holes of every knuckle portion of every hinge on the edge of a hinged member must be in alignment so that the hinges will not bind as they are rotatbly activated. When the knuckle portions are not mounted on common leaves, it is difficult to establish and maintain proper alignment. Considerable skill, effort and time are required to mount such separate hinges at spaced apart locations on a structure. In a conventional piano hinge, the problem of maintaining proper alignment between the knuckle portions is generally solved by providing knuckle or lug portions along the full length of the continuous hinge leaves so that the elongated channel defined between them when they are interposed with one another is substantially continuous. The requisite alignment is provided during the manufacturing of the hinge elements because all of the knuckles or lugs on one leaf are aligned at the time of manufacture, not during assembly to a hinged member.
As those skilled in the art realize, mass-produced plastic parts, such as continuous hinges are often economically made by an injection molding process. Such parts require very few, if any, machining or finishing operations, and very little material, if any, is wasted. In addition, the cycle time of molding and releasing the completed plastic articles, when low, can be extremely cost effective. However, one of the most significant costs involved in the injection molding process is in the manufacturing of the mold itself. The cost of the mold substantially increases when higher tolerances are required. In addition, when holes are required in the finished molded product, special axially moveable rods or pins must be provided to form such holes. This increases the initial and maintenance costs of the mold. This also increases the cost of molding each part since the pin must travel a considerable distance as it is inserted and removed for each cycle. The long mold pin must be translated axially in an out for substantially the full length of the piano hinge during each molding cycle. The longer the mold pin, the longer the insertion and removal time. The length of the cycle time, and the associated cost, thus increase as the length of the mold pin increases. Undesirably, a piano hinge features all of these cost increasing characteristics. Higher tolerances are required across numerous holes in the knuckles or lugs. If the mold pin is not accurately and positively supported, the resulting hinge will bind, which is unacceptable. The production of these holes requires that a long mold pin be accurately supported and linearly inserted and extracted for substantially the full length of the part with each molding cycle. This substantially increases the cycle time of the process. The costs and difficulties are further multiplied when, as is often required in mass production, multiple cavity molds are used to make a multitude of parts in one injection cycle. With shorter conventional hinges the mold pins are shorter, the molds are cheaper to make and maintain, and the cycle times are significantly lower. Thus, the costs associated with producing an injection molded long plastic piano hinge are typically higher than those to produce two conventional plastic hinges of the same combined length.
One previous expedient that has been proposed to avoid producing a continuous one piece plastic piano hinge is found in Krolick U.S. Pat. No. 4,651,382. In Krolick, a plurality of short solvent bondable plastic hinges are disclosed to replace a metal hinge. These short hinges are generally of a typical hinge configuration and are quite well known in the art to be cost effectively made by the injection molding process. One of the configurations in Krolick shows a piano hinge assembled from a plurality of the short plastic hinges attached in a chain. Each hinge pin of the plurality of short plastic hinges is offset so as to interlock with an adjacent hinge. The combination of the short hinges forms the piano hinge. This configuration, while it avoids the difficulties and expenses of injection molding one continuous leaf piano hinge, has its own disadvantages. For example, labor costs in assembly are undesirably substantially increased as each hinge pin must be offset and interconnected to an adjacent hinge. Errors in assembly can result in rejected parts, including the hinged members to which the hinges are mounted. Assembly time is undesirably increased since each hinge must be individually bonded to the object to be hinged. In addition, numerous separate parts must be made in order to produce just one piano hinge.
Those concerned with these problems recognize the need for an improved, more cost effective all plastic continuous hinge that is similar in function to a conventional piano hinge, that can be more easily mass produced by the injection molding process at the lowest possible cycle time, that requires a minimal amount of labor to assemble, and that uses a minimal amount of plastic material.
These and other difficulties of the prior art have been overcome according to the present invention.
A preferred embodiment of the elongated plastic hinge according to the present invention comprises two elongated hinge leaves, and two shortened knuckle or lug portions extending for less than the length of the leaves generally adjacent the opposed ends of the hinge leaves so as to provide an open spine with the alignment of a full spine. Each elongated hinge leaf has a body portion with at least two axially aligned and spaced apart lugs or knuckles projecting from an edge thereof. The hinge leaves also include an attachment surface adapted to allow the hinge to be solvent bonded to hinged members. Each lug contains an axially aligned pivot bore, and the lugs on one hinge leaf intermittently intermesh with lugs on the other hinge leaf to establish pivot connections once hinge pins are installed through the bores of each lug. Each set of intermeshed lugs forms one pivot connection. A measure of the openness of the hinge spine is provided by comparing the combined length of all of the pivot connections to the overall length of the hinge. The elongated plastic hinges according to the present invention have a pivot connection to hinge length ratio of less than about 0.8:1. It has been found that a pivot connection to hinge length ratio of between approximately 0.25-0.75:1 percent optimizes the material savings compared to a conventional piano hinge. According to one embodiment, a pivot connection to hinge length ratio of approximately 0.5:1 is preferred.
When more than two pivot connections are to be provided, it has been discovered that inexpensive snappedly engaging pivot portions provide particularly advantageous results. These snappedly engaging pivot portions reduce molding costs, reduce the amount of material used, and reduce assembly time. Unexpectedly, utilizing just two short hinge pins on just two pivot portions provides enough structural rigidity to prevent the hinge from separating during rough use where full piano hinges had customarily been used. These unique and desirous benefits significantly contribute to reducing the cost of the elongated plastic hinge so that they can be competitively mass produced by the injection molding process.
During the injection molding process a mass of molten thermoplastic plastic material is injected into a mold. The mold contains at least one cavity that is in the shape of a hinge leaf including the body and the lug portions. Where coaxially extending channels for receiving the pivot pins are to be located in the lug portions, the mold cavity includes a pair of spaced apart opposedly reciprocally mounted mold pins. The combined lengths of these mold pins are in the same proportions to the length of the hinge leaf shaped mold cavity as the pivot connections are to the length of the elongated hinge, that is, 0.8:1 or less. One molding cycle consists of closing the mold and reciprocating the mold pins toward one another to the full extend of their range of travel. In this fully inserted position they are disposed in a generally coaxial but spaced apart configuration. They are spaced apart by an amount that is at least equal to the open length of the spine of the hinge. The molten thermoplastic material is then injected into the hinge leaf shaped cavity and allowed to solidify. The mold pins are then reciprocated away from one another until they are completely withdrawn from the solidified hinge leaf. The mold is opened and the hinge leaf is ejected from the mold. Each of the mold pins travels less than one-half the length of the molded hinge leaf.
Other objects, advantages, and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.